Secret Key Cryptography by Frank Rubin

Author:Frank Rubin [Frank Rubin]
Language: eng
Format: epub
Publisher: Manning Publications
Published: 2022-08-09T00:00:00+00:00

CC

The ciphertext of block N is combined with the ciphertext of block N+1 after enciphering block N+1.

For the greatest strength, the chaining operation should be cumulative. First the chain vector from block N-1 is combined with block N. This result becomes the new chain vector, which is combined with block N+1. Chaining mode PP is the strongest, and chaining mode PC is second-best. Modes CP and CC are far weaker because Emily can see the chain vector. I recommend that modes CP and CC be used only with the combining functions xors, adds and poly.

Although modes CC and CP are weaker, there is an advantage to using them. In modes CC and CP it is not necessary to have a separate initialization vector. Sandra can use the last block of the plaintext message as the initialization vector. Riva can simply decipher the last block first. In fact, Riva can decipher any block without needing to decipher the block before. This can be valuable if the cipher uses indicators (section 14.3). Riva would decipher the indicator block first.

Let’s look at some stronger ways of chaining blocks.

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